1. Field of the Invention
The present invention relates generally to signature folding devices for web-fed printing presses and, more particularly, to a folder apparatus which allows on-the-fly adjustment of the folding operation to accommodate different press speeds and variable signature sizes.
2. Discussion of the Prior Art
In a conventional signature folder designed for use with web-fed newspaper or large publication presses, a number of folding stations are provided for producing a single, fixed-size product.
Typically, an upstream folder cuts the web into sheets and folds the web along a line extending in a direction transverse to the web. This cross-folded signature is then delivered onto a conveyor and can, if desired, be folded again along a line extending in a direction transverse to the cross fold line. The subsequent fold is formed by a chopper, which is a thin piece of spring steel that is lowered onto each signature as it travels along the conveyor, forcing the signature between two inwardly rotating rollers.
In order to properly align each signature with the chopper, it is known to provide a stationary head stop extending across the conveyor within the travel path of the signatures so that each signature strikes the stop and is squared relative to the chopper prior to the chop-folding operation so that it will be folded accurately.
If the head stop is not properly positioned along the conveyor, the signatures approaching the chopper will either be engaged by the chopper before striking the head stop, in which case the signatures are not always square, or strike the head stop with such force as to damage the signature, e.g. by bending the corners over or the like.
Thus, it is necessary to determine where the stop should be positioned along the conveyor in order to stop and square the signatures without damaging them. Unfortunately, once the head stop is adjusted for use at any particular press speed, it is not suitable for other press speeds. For example, if the stop is mounted in a position suitable for a given press speed, and the speed is increased, the signatures traveling on the conveyor have greater momentum, hitting the stop harder. Thus, damage to the signatures occurs unless the device is first shut downs and the position of the head stop is adjusted for the new press speed.
Another solution is to adjust the timing of the chopper movement relative to the moving signature so that the chopper engages the signature after the signature is squared but before the momentum of each signature carries it into the head stop with so much force as to cause damage to the signature. Although this solution reduces or eliminates down time of the press, it requires a special variable mechanical transmission and increases the cost of the press substantially.